Iron complexes semiquinones

The first reported low-spin iron(III) semiquinonate complex contained the 3,5-di-t-butyl-l, 2-benzoquinonate radical anion and a tetraazamacrocycle (tazm). It reacts reversibly with acetonitrile to give [Fe (tazm)(MeCN)2] plus 3,5-di-t-butyl-l,2-benzoquinone. " The related 1,2-iminobenzosemiquinonate (ibsq) complexes [Fe(ibsq)2X] have 5 = 5/2 for X = C1 and 5 =3/2 for X = I the complex with X = Br is a mixed spin (5=5/2, 3/2) species. " ... 

Fig. 8. Photochemical and chemical reduction of the semiquinone-iron complex in PS-II membranes (a) sample frozen in the dark, (b) sample illuminated for 10 m at 5 K, (c) sample illuminated for 10 m at 77 K, (d) sample illuminated for 4 m at 200 K. and (e) sample frozen in the dark in the presence of dithionite (no mediators present). EPR spectra were measured at 4.8 K. Figure source Rutherford and Mathis (1983) A relationship between the midpoint potential of the primary acceptor and low temperature photochemistry in photosystem II. FEES Lett 154 332.

NMR-shifts of the catecholate ligand in the complexes indicate a contribution from an iron(II)—semiquinone form. This is due to the covalency of the irondll )-catecholato bond, which is enhanced by the increasing Lewis acidity of the metal center. The radical character of the coordinated 3,5-DTBC correlates with the reactivity of the complexes. The authors [99] conclude that dioxygen attacks the coordinated semiquinone radical (Scheme 6) rather than first binding to... 

The mechanism shown in Scheme 5 postulates the formation of a Fe(II)-semi-quinone intermediate. The attack of 02 on the substrate generates a peroxy radical which is reduced by the Fe(II) center to produce the Fe(III) peroxide complex. The semi-quinone character of the [FeL(DTBC)] complexes is clearly determined by the covalency of the iron(III)-catechol bond which is enhanced by increasing the Lewis acidity of the metal center. Thus, ultimately the non-participating ligand controls the extent of the Fe(II) - semi-quinone formation and the rate of the reaction provided that the rate-determining step is the reaction of 02 with the semiquinone intermediate. In the final stage, the substrate is oxygenated simultaneously with the release of the FemL complex. An alternative model, in which 02 attacks the Fe(II) center instead of the semi-quinone, cannot be excluded either. 

Pyridine nucleotide-dependent flavoenzyme catalyzed reactions are known for the external monooxygenase and the disulfide oxidoreductases However, no evidence for the direct participation of the flavin semiquinone as an intermediate in catalysis has been found in these systems. In contrast, flavin semiquinones are necessary intermediates in those pyridine nucleotide-dependent enzymes in which electron transfer from the flavin involves an obligate 1-electron acceptor such as a heme or an iron-sulfur center. Examples of such enzymes include NADPH-cytochrome P4S0 reductase, NADH-cytochrome bs reductase, ferredoxin — NADP reductase, adrenodoxin reductase as well as more complex enzymes such as the mitochondrial NADH dehydrogenase and xanthine dehydrogenase. 

Oxidation of UQH2 by the iron-sulfur protein generates the semiquinone UQH , which then serves as the reductant for cytochrome bL. The reduced iron-sulfur protein transfers an electron to cytochrome cx and on to cytochrome c. Meanwhile, the reduced cytochrome bL passes an electron to cytochrome bu, which then contributes the electron for reduction of another molecule of UQ at a second site in the complex. 

A mechanism for extradiol cleavage is proposed in Figure 18 [5,158], Substrate binds first to the iron(II) center, followed by 02, to form a ternary complex akin to the ES—NO complex described earlier. Electron transfer from metal to 02 in the Fe(II)—02 adduct results in a superoxide-like moiety and imparts nucleophilic character to the bound 02. This in turn generates semiquinone character on the bound substrate, which is attacked by the nascent superoxide to form a peroxy intermediate that decomposes by a Criegee-type rearrangement to the observed product. 

Although the uncharged tris(3,5-di-t-butylcatecholate) complex of iron [Fe(DTBC)3] has been extensively studied, " the proposed bonding in these reports is unclear. The most common formulation is as an ionic salt between iron(lll) and three semiquinone anion radicals, Fe +(DTBSQ 03 However, the magnetic moment is 2.9 BM (consistent with an 5 = 2/2 spin state) and the electrochemistry indicates a ligand-centered reduction. Both of these characteristics are analogous to ferrate dianion. 

The primary acceptor in PS II is a plastoquinone, PQ, as ascertained from optical absorbance difference spectroscopy [46], Until recently, the EPR spectrum of the semiquinone escaped observation, and only the advent of preparation methods for PS II subchloroplast particles made its recording possible. As surmised earlier, the spectrum of the intact acceptor [47] very much resembled the very broad qui-none-iron acceptor complex in purple bacteria, whereas in iron-depleted PS II particles the narrow spectrum typical of an immobilized semiquinone was found [48], As in the bacterial photosystem, flash-induced reduction of Q, of the second quinone, Qb, or of both resulted in somewhat different EPR spectra, indicative of structural changes that influence the magnetic interaction between the semiquinone and the non, and/or between the two semiquinones [49],... 

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